Impedance tuners are used to present impedances (also referred to as VSWR, gamma, reflection, return loss) to a device under test and measure its performance as a function of said impedance. Various forms of impedance tuners exist, including mechanical slide-screw tuners, which use a mechanical probe in conjunction with a slabline, and whose probe position dictates the resulting impedance. The probe's horizontal position in relation to the slabline usually affects the phase of the impedance presented to the Device under Test (DUT), whereas the probe's vertical position usually affects the magnitude of the impedance presented to the DUT. Because the probe can be moved in extremely small steps with high resolution, thousands, tens of thousands or even hundreds of thousands of impedance states can be generated at will. Advantages include high resolution, high point count, high power handling and wide frequency bandwidth. Disadvantages include tuning speed and size and weight.
Electronic impedance tuners utilize PIN diodes or switches, with fixed or swept bias voltages and currents, which result in a non-uniform low-density impedance grid. Examples of the electronic or solid state impedance tuners are described in U.S. Pat. Nos. 5,034,708 and 5,276,411, the entire contents of which patents are incorporated herein by this reference. Advantages of electronic or solid state tuners include high tuning speed and small size and weight. Disadvantages include low point density and low power handling.
It is important to note that high tuning speed and high point density are two of the primary requirements of an impedance tuner.
Passive load pull systems have been widely used to characterize microwave devices. Load pull systems measure a DUT under controlled conditions, including controlled impedances seen by the DUT. The controlled impedances may include the impedance on any port of the DUT, and a typical load pull measurement would measure the DUT performance at multiple impedances to show the effect of impedance on the DUT performance. Some other conditions that may be controlled and/or varied include frequency, power level, bias values, or temperature.
In this document, impedance, reflection, or reflection coefficient are all used as general terms to describe the RF termination seen at an RF port. They are functions of the signal coming out of an RF port and the signal at the same frequency coming into the port. Reflection coefficient is related to impedance by the expression
      Z    =                  Z        0            ⁢                          ⁢                        (                      1            +            Γ                    )                          (                      1            -            Γ                    )                      ,
where Z is the impedance and r is reflection coefficient. Both terms contain the same information, so that if one is known, the other is also known. Therefore, in this document they will be used interchangeably.
Tuning resolution is a term that indicates how close the available impedances are to each other in the impedance or reflection plane. High resolution (or fine resolution) means that the gaps between available impedances are very small. Lower resolution means that the gaps between available impedances are larger.
Matching range is a term that indicates the maximum reflection that can be achieved by at tuner over a desired phase range. As an impedance tuner specification, this generally means for all or most reflection phase values (allowing for tuning resolution). However, this definition used for a tuner system can be modified to mean the maximum reflection over a limited phase range. For example, many power transistors require low impedances at the fundamental frequency, so a high matching range is only needed at the low impedance (left side) of the Smith chart.
Automated load pull systems have widely used mechanical tuners, although the size and slow speed is a limitation. Solid state tuners using PIN diodes as switching elements have also been used for load pull systems, and these may provide a big speed advantage, but they have limited tuning resolution.